As conventional force sensors, a strain gauge, a capacitive force sensors, a conductive rubber, and the like are developed. Each of the force sensors using those utilizes deformation of a member, and thus it is necessary to secure a sufficient amount of the deformation for realizing high sensitivity and a wider dynamic range of measurement. Therefore, if the sensor is downsized, sufficient deformation amount cannot be obtained. As a result, there arises a problem that an output signal is buried in noise to deteriorate accuracy.
As a countermeasure against the problem, there is provided a sensor utilizing a resonance phenomenon of a piezoelectric body as described in Japanese Patent Application Laid-Open No. S52-078483. The piezoelectric body has a property of vibrating in a specific direction when a voltage which temporally changes (e.g., alternating voltage) is applied, according to time variation of amplitude of the voltage.
According to the technique described in Japanese Patent Application Laid-Open No. S52-078483, an impedance 6 is connected in series to the piezoelectric body first, and a voltage having a fixed frequency that is not higher than a resonance frequency of a piezoelectric body 1 is applied from a power supply 7, whereby the piezoelectric body is vibrated. If a force is exerted on the piezoelectric body in this state, the impedance of the piezoelectric body increases so that the resonance frequency of the piezoelectric body is shifted to a high frequency side. In the shifted state, a value of the voltage applied to the piezoelectric body 1 is changed from the value before the force is exerted. According to this change of the voltage value, magnitude of the force that has exerted on the piezoelectric body can be quantitatively calculated (see FIG. 8).
The piezoelectric vibration type force sensor described in Japanese Patent Application Laid-Open No. S52-078483 has an advantage that its sensitivity is high with a low load because variation of the voltage amplitude is large. However, as an external force exerted on the sensor increases, the variation of the voltage amplitude decreases. Thus, its sensing range is narrow.
As a countermeasure against this problem, there are proposed methods for expanding the sensing range in Japanese Patent Application Laid-Open No. S60-187834 and Japanese Patent Application Laid-Open No. S60-222734.
First, Japanese Patent Application Laid-Open No. S60-187834 is characterized in that a structure in which two piezoelectric bodys are bonded to each other, i.e., a so-called bimorph type piezoelectric body is used. One end of this bimorph piezoelectric body is fixed while the other end is made to be a free end, whereby a force is exerted on the piezoelectric body. In addition, a frequency signal higher than a resonance point of the piezoelectric body is applied to the same. This sensor has the structure in which two piezoelectric bodys are bonded to each other, and hence amplitude thereof can be made larger, with the result that the range of sensing forces can be expanded.
Next, as to Japanese Patent Application Laid-Open No. S60-222734, a phase angle difference between the voltage applied to the piezoelectric body and a terminal voltage of the piezoelectric body, which is caused by the external force, is sensed to control the frequency so that the phase angle difference becomes a set value. This frequency variation is utilized for sensing the force. Thus, the sensitivity can be uniform and the sensing range can be made wider.
However, the invention of Japanese Patent Application Laid-Open No. S60-187834 uses the bimorph piezoelectric body. The bimorph piezoelectric body has the structure in which two piezoelectric elements are bonded to each other, and hence it is disadvantageous for cost reduction and downsizing.
In addition, the invention of Japanese Patent Application Laid-Open No. S60-187834 senses the phase angle difference between the voltage applied to the piezoelectric body and the terminal voltage of the piezoelectric body, which is caused by the external force, thereby controlling the frequency so that the phase angle difference becomes a set value. However, an additional phase angle sensing circuit and an additional phase angle setting circuit are necessary, resulting in complication of the circuit. Therefore, it is also disadvantageous for cost reduction and downsizing.